There are 99 counties in the U.S. state of Iowa. The first two counties, Des Moines County and Dubuque County, were created in 1834 when Iowa was still part of the Michigan Territory. In preparation for Michigan's statehood, part of Michigan Territory was formed into Wisconsin Territory in 1836.[1] Two years later, the western portion was split off to become Iowa Territory.[2] The south-eastern part of Iowa Territory became Iowa, the 29th state in the union, on 28 December 1846,[3] by which point 44 counties had been created. Counties continued to be created by the state government until 1857, when the last county, Humboldt County, was created.[4] One of the most significant days in Iowa county history was January 15, 1851, on which 49 counties were created.[5]

The Iowa Constitution of 1857, which is still in effect today, states that counties must have an area of at least 432 square miles (1,120 km2), nor can any county be reduced below that size by boundary changes.[6] However, exceptions to this rule were granted, as ten counties have areas below this size. (The table below shows land area, but the Constitution deals with total area.[6]) The smallest county (Dickinson) has a land area of 381 sq mi (990 km2), while the largest (Kossuth) has an area 973 sq mi (2,520 km²). Polk County is the most densely populated county at 756/sq mi (291.7/km2), an increase in density from 2000 when it was 657 inhabitants per square mile (254/km2).[7] Polk County contains the state's capital and largest city, Des Moines. In addition Iowa has one of the fewest number of counties whose boundaries are dictated by natural means, the vast majority of which being formed by lines of survey instead, resulting in a large number of "box counties".

The number in the column headed "#" is the one used on the map from the National Atlas of the United States, shown on the left. The Federal Information Processing Standard (FIPS) code, which is used by the United States government to uniquely identify counties, is provided with each entry. The FIPS code for each county links to census data for that county.

^"Henry County, Iowa". Retrieved 2008-08-17. According to most sources, Henry County (440 square miles) was named for Henry Dodge (1782-1867), who served in the Black Hawk War and other Indian wars and governor of Wisconsin Territory (1836-1841); others credit the county’s name to General James Dougherty Henry (1797-1834), a hero of the Black Hawk War.

^"Louisa County, Iowa". Official website. Retrieved 2008-08-17. According to some authorities, the county was named for Louisa Massey, who gained fame during her time because she avenged the murder of her brother by shooting the man responsible. Other historians claim the name was given in honor of Louisa County, Virginia. There are 418 square miles (1,080 km2) in Louisa County.

^John Adams, Tama County Auditor (2002). "Tama County, Iowa". e-referencedesk.com. Clarksville, VA: Web Marketing Services, Inc. LLC. Retrieved 2008-10-02. The origin of the naming of Tama County is somewhat disputed. Some authorities say the name comes from Taimah, a Fox Indian Chief. Others say it comes from Taomah, wife of Poweshiek--another Fox Indian Tribe Chief. Still others say the county is named after the Indian Chief Potama or Pottama.